1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor for use in image forming by an electrophotographic process, and a process cartridge and an electrophotographic apparatus using the electrophotographic photoreceptor.
2. Description of the Related Art
Electrophotographic processes, which allow high-speed and high-quality printing, have been used in various electrophotographic apparatuses such as copying machines, laser beam printers, and the like.
Mainstream electrophotographic photoreceptors used in electrophotographic apparatuses use an organic photoconductive material, and the performance of the photoreceptors has been improved as more electrophotographic apparatuses adopt a functionally separated layered electrophotographic photoreceptor that has a charge-transporting material and a charge-generating material formed in separate layers.
Currently, in the case of layered electrophotographic photoreceptors, an undercoat layer is first formed on an aluminum substrate and a photosensitive layer consisting of a charge-generating layer and a charge-transporting layer is formed thereon.
The operational and environmental stability of electrophotographic photoreceptors during repeated use largely depend not only on the charge-generating layers and charge-transporting layers but also on the undercoat layer, and there exists a need for an undercoat layer that does not accumulate electric charge through repeated use.
The undercoat layer also plays a significant role in preventing image quality defects, and thus is an important functional layer for suppression of defects and stains on a substrate or image quality defects derived from defects and surface irregularity of upper layers such as the charge-generating layer.
In particular, in electrostatic charging devices using a contact charging process, which generate a smaller amount of ozone and are thus recently in electrophotographic apparatuses enjoying wider use in place of corotron-charging devices, electrical pinholes are often generated by a locally high electric field applied onto locally degraded areas of the electrophotographic photoreceptor during contact charging, leading to occasional image quality defects.
The pinhole leak is not only caused by the defects of coated films on an electrophotographic photoreceptor itself as described above, but also by conductive foreign substances generated in an electrophotographic apparatus, which come into contact with or penetrate into the electrophotographic photoreceptor, forming conductive paths between the contact-type electrostatic charging device and the electrophotographic photoreceptor substrate. In some special cases, foreign substances derived from other members of the electrophotographic apparatus and dust brought into the electrophotographic apparatus generate leak points from the contact-type electrostatic charging device, by sticking into the electrophotographic photoreceptor.
As a method to overcome the above problems, a layer containing conductive fine powders has been formed on the substrate to conceal such defects and stabilize the electrical properties of the substrate by thickening the undercoat layer.
An example thereof is a method for forming a conductive layer containing dispersed conductive powders on an aluminum substrate and additionally forming an undercoat layer over the conductive layer. In such a case, the conductive layer conceals the defects and also adjusts the resistance of the substrate, while the undercoat layer demonstrates a blocking (charge injection-controlling) function.
Another example is a method of applying a conductive powder dispersion layer having both a blocking (charge injection-controlling) capability and a resistance-adjusting capability on a substrate and using it as an undercoat layer having the functions both of a blocking (charge injection-controlling) layer and a resistance-adjusting layer.
The latter kind of undercoat layer, which permits elimination of one layer from the layers in the former kind of undercoat layer, allows simplification of the manufacturing process for the electrophotographic photoreceptor and cost reduction.
However, the latter kind of undercoat layer, which demands both the resistance-adjusting and charge injection-controlling functions in a single undercoat layer, imposes a significant restriction on material design.
In addition, for leak prevention, a thicker undercoat layer is more effective and thus a film having a thickness of 10 μm or more is demanded; but if the resistance of a thick film is reduced for obtaining favorable electrical properties, it tends to have a deteriorated charge-blocking property and an increase in background fog as an image quality defect.
The thickness of the latter kind of undercoat layer so far commercialized, for example, undercoat layers containing conductive titanium oxide powders, remains approximately in the range of one to several μm; and thus, there has existed no thickened undercoat layers containing conventional materials that satisfy all the requirements for electrophotographic photoreceptors such as improved leakage resistance, stabilized electrical properties, and low background fog.
In particular, recent increased recognition of environmental issues has urged prompt development of longer-life electrophotographic photoreceptors, and under these circumstances, stabilization of electrical properties and image quality during long-term repeated use is essential.
Alternatively, methods of adding additives such as an electron-accepting material and an electron-transporting material to the undercoat layer has been proposed (e.g., Japanese Patent Application Laid-Open (JP-A) Nos, 7-175249, 8-44097, and 9-197701).
However, even with these methods, it is impossible to obtain a thickened undercoat layer that satisfies all the requirements for electrophotographic photoreceptors such as the improved leakage resistance, stabilized electrical properties, and low background fog.
The present invention has been devised in view of these problems, and provides an electrophotographic photoreceptor superior in electrical properties that has a smaller fluctuation in electrical properties and generates fewer image quality defects such as pinhole leaks and the like during repeated use, and a process cartridge and an electrophotographic apparatus using the electrophotographic photoreceptor.